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Why does a passing sound change pitch?

A passing siren can seem to drop in pitch even if its source keeps producing the same note. Motion changes how frequently its sound waves reach a listener. The cleanest version of this puzzle uses a steady tone, a stationary listener, and a source moving below the speed of sound in still air.

3–5 min

Questions you could explore

  • What changes between the approaching and departing sound?
  • How is a pitch change different from a loudness change?

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How can a ship made of steel float?

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Why does a passing sound change pitch? | Speech Topic | speaking.app